Liquid crystal displays are widely utilized for a monitor of a personal computer and a portable equipment, and for televisions, because of various advantages such as a low voltage drive, a lower electric power consumption and possibility for a compactification and a thin structure. Such liquid crystal displays are proposed in various modes depending on the liquid crystal molecules within a liquid crystal cell, but a TN mode, in which the liquid crystal molecules are twisted by about 90° from a lower substrate to an upper substrate of the liquid crystal cell, has been employed principally.
In general, the liquid crystal display is constituted of a liquid crystal cell, an optical compensation sheet, and a polarizer. The optical compensation sheet is used for removing a coloration on the image or expanding a view angle, and a stretched birefringent film or a transparent film coated with a liquid crystal material is used for this purpose. For example JP-A-62-210423 discloses a technology of applying an optical compensation film, which is prepared by coating, aligning and fixing a discotic liquid crystal on a triacetyl cellulose film, to a TN-mode liquid crystal cell, thereby expanding a viewing angle. However, a liquid crystal display for a television use, having a large image size and anticipated for observation from various angles, involves strict requirements for the viewing angle dependence of the image, and even the aforementioned method is unable to meet such requirements. For this reason, liquid crystal displays different from the TN mode are being investigated, such as those of an IPS (in-plane switching) mode, an OCB (optically compensatory bend) mode and a VA (vertically aligned) mode. In particular, the VA mode is attracting attention for use in television, because of a high contrast and a relatively high production yield.
However the VA mode, though being capable of displaying an almost complete black color in a normal direction to the panel, causes a light leakage when the panel is observed from an inclined direction, and thus results in a limited viewing angle. In order to avoid such drawback, it is proposed to position a retardation plate, having a refractive index anisotropy in which a refractive index in a film thickness direction is sufficiently smaller than a refractive index in an in-plane direction, between the liquid crystal layer and at least one of the polarizing plates (for example JP-A-62-210423). It is also proposed to reduce the light leakage, by utilizing, in combination, a first retardation plate having a positive monoaxial refractive index anisotropy and a second retardation plate having a negative refractive index anisotropy in which the refractive index in the film thickness direction is sufficiently smaller than the refractive index in the in-plane direction (for example Japanese Patent No. 3027805). It is further proposed to improve the viewing angle characteristics of a VA-mode liquid crystal display, utilizing an optically biaxial retardation plate, having refractive indexes different in three-dimensional directions of a film (for example Japanese Patent No. 3330574).
On the other hand, also in the IPS mode, a slight light leakage in a diagonally inclined incident direction in a black display state is recognized as a cause of deterioration in the display quality. For improving the displayed color and the viewing angle in the black display state, it is being considered, also in the IPS mode, to provide an optical compensation material with birefringent characteristics, between the liquid crystal layer and the polarizing plate. It is disclosed, for example, that the coloration of the image, when a white display or a display of an intermediate tone is observed from an inclined direction, can be improved by positioning a birefringent medium, having mutually orthogonal optical axes and capable of compensating a change in the retardation of the liquid crystal layer in an inclined position, between a substrate and a polarizing plate (see JP-A-9-80424). There are also disclosed a method of utilizing an optical compensation film, formed by a styrenic polymer or a discotic liquid crystalline compound, having a negative intrinsic birefringence (see JP-A-10-54982, JP-A-11-202323 and JP-A-9-292522), a method of combining, as an optical compensation film, a film having a positive birefringence and having an optical axis in the plane of the film and a film having a positive birefringence and having an optical axis in a normal direction to the film (see JP-A-11-133408), a method of utilizing a biaxial optical compensation sheet with a retardation of a half wavelength (see JP-A-11-305217), and a method of utilizing a film having a negative retardation as a protective film of a polarizing plate and providing a surface of such film with an optical compensation layer having a positive retardation (see JP-A-10-307291). Also disclosed is an invention of utilizing a retardation film having Nz of from 0.4 to 0.6 and an in-plane retardation of from 200 to 350 nm thereby suppressing a light leakage, caused by an aberration in a crossing angle of the polarizing axes from an orthogonal relationship, experienced when orthogonally disposed polarizing plates are observed from an inclined direction (see JP-A-2004-4642).
However, the aforementioned methods reduces the light leakage only in a certain wavelength region (for example green light around 550 nm), and do not take into consideration the light leakage in other wavelength regions (for example blue light around 450 nm and red light around 650 nm). Therefore, so-called color shift phenomenon, in which a black display is colored blue or red when observed from an inclined direction, has not been resolved.